RFID Antenna Array For Gaming

ABSTRACT

An RFID system includes multiple antennas and uses amplitude and phase information of the RFID signals received by each antenna to determine the position of RFID tags in the vicinity. More than one antenna can receive the RFID signals during a single read cycle, enabling the RFID system to operate more quickly than a system that energizes antennas separately.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent application Ser. No. 15/814,170 filed Nov. 15, 2017 for “RFID Antenna Array For Gaming”, which is incorporated herein by reference.

BACKGROUND

The present invention relates to gaming, and in particular, to a radio frequency identification (RFID) system with an antenna array for detecting the locations of RFID tags on a gaming table.

Unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.

Tracking the location of gaming tokens in real-time on a gaming table has the potential to revolutionize the gaming industry by providing cash management and improved security. Tying this data to specific players allows casinos to create accurate player profiles while simultaneously alleviating the pit boss of mundane tasks that take years of training to hone.

Traditional RFID systems have tried to address the gaming market with limited success. In a typical RFID system, the excitation antenna defines a “working volume” within which the energy projected by the antenna is sufficient to power the RFID tag. This “working volume” is generally poorly defined with the only option to increase/decrease power to adjust the read range. But doing so extends the read range in ALL directions, introducing cross-talk errors when multiple antennas are in close proximity. Existing products on the market suffer from multiple shortcomings. First, they are limited to discrete betting spots. Second, they are limited in the chip stack heights they can read. Third, they have very poor discrimination between adjacent betting spots. Fourth, they have higher than acceptable read errors. Fifth, they have slow read rates that miss important events (e.g., placement and removal of chips, etc.).

These shortcomings limit the available technology to games where the betting spots are widely separated (e.g. a single “pot”), to detecting initial bets only (not capturing transient events such as payouts), and identifying counterfeit tokens only prior to their use on a table (not during gameplay).

U.S. Application Pub. No. 2013/0233923 discusses a ferrite core technology. The ferrite core technology overcomes many of the above-noted shortcomings, but does not address the need to track multiple separate bets placed by different bettors on a single larger betting spot (such as when “back bettors” share a betting spot with seated bettors on traditional Baccarat “racetrack” layouts). Also needed is the ability to discriminate the location of very closely spaced bets (such as can be found on a roulette table).

U.S. Application Pub. No. 2017/0228630 discusses a solution involving two intersecting antenna arrays. One array of horizontal antennas provides one coordinate, and a second array of vertical antennas provides a second coordinate. Signal strength information comparing adjacent antennas may then be used to interpolate a higher fidelity set of coordinates.

Although the approach of U.S. Application Pub. No. 2017/0228630 does work, it suffers from the simple fact that reading RFID tags takes time—and reading tags multiple times for purposes of interpolation multiplies the required time such that capturing an accurate “snapshot” of transient events with large numbers of tags may not practical in certain gaming environments.

The typical RFID system addresses the question, “Who's there?” The response is a series of unique item identifiers (e.g., serial numbers). As discussed above, the ferrite core technology discussed in U.S. Application Pub. No. 2013/0233923 is directed to addressing the additional question “Where are you?” as a way to track individual bets.

U.S. Application Pub. No. 2016/0217645 discusses using a network analyzer device prior to an RFID read, thereby being able to direct the RFID reader to only those antennas with tags present. This describes a serial approach that eliminates the “overhead” of looking for tags using an RFID reader where none are present, as using the network analyzer device takes less time than using the RFID reader.

Both U.S. Application Pub. No. 2013/0233923 and U.S. Application Pub. No. 2016/0217645 involve the placement of bets in specific areas (the betting spots). RFID tags not placed in one of the defined areas will not be read correctly. Neither of these disclosures addresses the need to detect bets placed anywhere on a larger bounded area. The additional disclosure of U.S. Application Pub. No. 2017/0228630 does address placing multiple bets within a larger bounded area. However, the system disclosed therein involved multiple RFID reads to define the coordinates of each bet, which is a time consuming process.

All three of U.S. Application Pub. No. 2013/0233923, U.S. Application Pub. No. 2016/0217645 and U.S. Application Pub. No. 2017/0228630 describe systems to identify and locate RFID tags by using signal strength information as measured by the RFID reader to determine proximity to a specific antenna. U.S. Application Pub. No. 2013/0233923 describes a system that increases the signal strength at the proper antenna, which further improves accuracy.

SUMMARY

One issue with existing systems that use an array of antennas to locate a tag within the array is the time involved in energizing each antenna, in order to read the RFID tags in the vicinity of each antenna and then repeat this process for each subsequent antenna. There is a need for a faster method to accurately locate and track individual closely spaced bets that can be placed anywhere within a defined boundary on a gaming table. There is a need for increased speed in a system that applies game rules to calculate the amounts and positions of the original bets, and also to correlate transient events such as payouts to winning bets.

Given the above, embodiments are directed toward using phase information of the detected RFID signals in order to improve the operation of the system.

According to an embodiment, a system determines the locations of objects in a gaming environment. The system includes a main antenna associated with an area on a gaming table, a first plurality of antennas oriented in a first direction and associated with the area on the gaming table, a second plurality of antennas oriented in a second direction, a main radio frequency identification (RFID) transmitter coupled to the main antenna, a main RFID receiver coupled to the main antenna, a first plurality of RFID receivers coupled to the first plurality of antennas, a second plurality of RFID receivers coupled to the second plurality of antennas, and a controller that controls the main RFID transmitter to generate an RFID inventory command. The second direction differs from the first direction, the second plurality of antennas overlaps the first plurality of antennas, the first plurality of antennas and the second plurality of antennas intersect at a plurality of locations within the area, and each of a plurality of RFID tags in the area responds to the RFID inventory command according to an anti-collision process. In response to the RFID inventory command, the main RFID receiver receives a first plurality of responses from the plurality of RFID tags, the first plurality of RFID receivers receives a second plurality of responses from the plurality of RFID tags, and the second plurality of RFID receivers receives a third plurality of responses from the plurality of RFID tags. The controller determines an identifier for each of the plurality of RFID tags using at least one of the first plurality of responses, the second plurality of responses, and the third plurality of responses, and the controller determines a position of each of the plurality of RFID tags by correlating amplitude information and phase information of the first plurality of responses, amplitude information and phase information of the second plurality of responses, and amplitude information and phase information of the third plurality of responses.

For a particular RFID tag of the plurality of RFID tags, the controller may simultaneously determine the identifier and the position of the particular RFID tag.

The RFID inventory command may be a single RFID inventory command that results in the controller determining the identifiers and the positions of all the plurality of RFID tags.

The first plurality of antennas and the second plurality of antennas may be overlapping and intersecting to define the position of each of the plurality of RFID tags in two dimensions within the area. The first plurality of antennas and the second plurality of antennas may intersect orthogonally and define the position of each of the plurality of RFID tags in an x dimension and a y dimension within the area.

The first plurality of antennas and the second plurality of antennas may define the position of each of the plurality of RFID tags using polar coordinates within the area.

The first plurality of antennas may be formed as a first non-overlapping, single layer, and the second plurality of antennas may be formed as a second non-overlapping, single layer. The first plurality of antennas may be formed as an overlapping, dual layer.

The controller may determine the position of each of the plurality of RFID tags using interpolation of the amplitude information of the second plurality of responses and the amplitude information of the third plurality of responses.

The controller may determine that a subset of the plurality of RFID tags are grouped together when the position of each RFID tag of the subset is within a defined range of at least one other RFID tag of the subset.

The controller may determine that a first subset of the plurality of RFID tags corresponds to a bet, and that a second subset of the plurality of RFID tags corresponds to a payout associated with the bet, according to the position of the first subset and the position of the second subset.

The controller may determine the identifier for each of the plurality of RFID tags using at least one of the second plurality of responses and the third plurality of responses.

The controller may use the first plurality of responses as reference information to normalize the second plurality of responses and the third plurality of responses. The controller may use the amplitude information of the first plurality of responses to normalize the amplitude information of the second plurality of responses and the amplitude information of the third plurality of responses.

The controller may use the phase information of the first plurality of responses to determine relative phase information for the second plurality of responses and relative phase information for the third plurality of responses.

When a first set of the plurality of RFID tags are associated with a first position, and when a second set of the plurality of RFID tags are associated with a second position, the controller may determine that the first set and the second set are a group when the first position and the second position are within a threshold distance.

According to an embodiment, a system determines the locations of objects in a gaming environment. The system includes a main antenna associated with an area on a gaming table, a first plurality of antennas oriented in a first direction and associated with the area on the gaming table, a second plurality of antennas oriented in a second direction, a main radio frequency identification (RFID) transmitter coupled to the main antenna, a main RFID receiver coupled to the main antenna, a first plurality of RFID receivers coupled to the first plurality of antennas, a second plurality of RFID receivers coupled to the second plurality of antennas, and a controller that controls the main RFID transmitter to generate an RFID inventory command. The second direction differs from the first direction, the second plurality of antennas overlaps the first plurality of antennas, and the first plurality of antennas and the second plurality of antennas intersect at a plurality of locations within the area. Each of a plurality of RFID tags in the area responds to the RFID inventory command according to an anti-collision process. In response to the RFID inventory command, the main RFID receiver receives a first plurality of responses from the plurality of RFID tags, the first plurality of RFID receivers receives a second plurality of responses from the plurality of RFID tags, and the second plurality of RFID receivers receives a third plurality of responses from the plurality of RFID tags. The controller determines an identifier for each of the plurality of RFID tags using at least one of the first plurality of responses, the second plurality of responses, and the third plurality of responses. The controller determines a position of each of the plurality of RFID tags by correlating amplitude information of the first plurality of responses, amplitude information and phase information of the second plurality of responses, and amplitude information and phase information of the third plurality of responses.

The details of this embodiment may otherwise be similar to the details of the previous embodiment.

According to an embodiment, a method determines the locations of objects in a gaming environment. The method includes generating, by a main radio frequency identification (RFID) transmitter coupled to a main antenna, an RFID inventory command, where the main antenna is associated with an area on a gaming table. The method further includes responding, by each of a plurality of RFID tags in the area, to the RFID inventory command according to an anti-collision process. The method further includes receiving, by a main RFID receiver coupled to the main antenna, a first plurality of responses from the plurality of RFID tags in the area in response to the RFID inventory command. The method further includes receiving, by a first plurality of RFID receivers coupled to a first plurality of antennas, a second plurality responses from the plurality of RFID tags in response to the RFID inventory command, where the first plurality of antennas is oriented in a first direction and is associated with the area on the gaming table. The method further includes receiving, by a second plurality of RFID receivers coupled to a second plurality of antennas, a third plurality of responses from the plurality of RFID tags in response to the RFID inventory command, where the second plurality of antennas is oriented in a second direction that differs from the first direction, where the second plurality of antennas overlaps the first plurality of antennas, and where the first plurality of antennas and the second plurality of antennas intersect at a plurality of locations within the area. The method further includes determining, by a controller, an identifier for each of the plurality of RFID tags using at least one of the first plurality of responses, the second plurality of responses, and the third plurality of responses. The method further includes determining, by the controller, a position of each of the plurality of RFID tags by correlating amplitude information of the first plurality of responses, amplitude information and phase information of the second plurality of responses, and amplitude information and phase information of the third plurality of responses.

The step of determining the position of each of the plurality of RFID tags may further include determining, by the controller, the position of each of the plurality of RFID tags by correlating the amplitude information and phase information of the first plurality of responses, the amplitude information and phase information of the second plurality of responses, and the amplitude information and phase information of the third plurality of responses.

The details of this embodiment may otherwise be similar to the details of the previous embodiments.

According to an embodiment, a system determines the locations of objects in a gaming environment. The system includes a main antenna associated with an area on a gaming table, a first plurality of antennas oriented in a first direction and associated with the area on the gaming table, and a second plurality of antennas oriented in a second direction, a main radio frequency identification (RFID) transmitter coupled to the main antenna, a first plurality of RFID receivers coupled to the first plurality of antennas, a second plurality of RFID receivers coupled to the second plurality of antennas, and a controller. The second direction differs from the first direction, the second plurality of antennas overlaps the first plurality of antennas, and the first plurality of antennas and the second plurality of antennas intersect at a plurality of locations within the area. The controller controls the main RFID transmitter to generate an RFID inventory command, where each of a plurality of RFID tags in the area responds to the RFID inventory command according to an anti-collision process. In response to the RFID inventory command, the first plurality of RFID receivers receives a first plurality of responses from the plurality of RFID tags, and the second plurality of RFID receivers receives a second plurality of responses from the plurality of RFID tags. The controller determines an identifier for each of the plurality of RFID tags using at least one of the first plurality of responses and the second plurality of responses. The controller determines a position of each of the plurality of RFID tags by correlating amplitude information of the first plurality of responses, and amplitude information of the second plurality of responses.

The controller may determine the position of each of the plurality of RFID tags by correlating the amplitude information and phase information of the first plurality of responses, and the amplitude information and phase information of the second plurality of responses.

The details of this embodiment may otherwise be similar to the details of the previous embodiments.

According to an embodiment, a method determines the locations of objects in a gaming environment. The method includes generating, by a main radio frequency identification (RFID) transmitter coupled to a main antenna, an RFID inventory command, where the main antenna is associated with an area on a gaming table. The method further includes responding, by each of a plurality of RFID tags in the area, to the RFID inventory command according to an anti-collision process. The method further includes receiving, by a first plurality of RFID receivers coupled to a first plurality of antennas, a first plurality responses from the plurality of RFID tags in response to the RFID inventory command, where the first plurality of antennas is oriented in a first direction and is associated with the area on the gaming table. The method further includes receiving, by a second plurality of RFID receivers coupled to a second plurality of antennas, a second plurality of responses from the plurality of RFID tags in response to the RFID inventory command, where the second plurality of antennas is oriented in a second direction that differs from the first direction, where the second plurality of antennas overlaps the first plurality of antennas, and where the first plurality of antennas and the second plurality of antennas intersect at a plurality of locations within the area. The method further includes determining, by a controller, an identifier for each of the plurality of RFID tags using at least one of the first plurality of responses, and the second plurality of responses. The method further includes determining, by the controller, a position of each of the plurality of RFID tags by correlating amplitude information of the first plurality of responses, and amplitude information of the second plurality of responses.

The step of determining the position of each of the plurality of RFID tags may include determining, by the controller, the position of each of the plurality of RFID tags by correlating the amplitude information and phase information of the first plurality of responses, and the amplitude information and phase information of the second plurality of responses.

The details of this embodiment may otherwise be similar to the details of the previous embodiments.

The following detailed description and accompanying drawings provide a further understanding of the nature and advantages of embodiments of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an RFID system 100.

FIG. 2 is a flowchart of a method 200 of operating an RFID system (e.g., the RFID system 100 of FIG. 1).

FIG. 3 is a graph showing a plot 300 of amplitude and phase information detected by one of the antennas 104 or 106 (see FIG. 1) as the chip is moved across it.

FIG. 4 is a graph showing plots 400 and 402 of amplitude and phase information detected by two of the antennas 104, or by two of the antennas 106 (see FIG. 1).

FIG. 5 is an overhead view of the antennas 104 and 106 of FIG. 1.

FIG. 6 is a block diagram of an RFID system 600. The RFID system 600 shows a specific implementation of the RFID system 100 (see FIG. 1).

FIG. 7 is a block diagram of a receiver 700.

FIG. 8 is a block diagram of an RFID system 800.

FIG. 9 is an overhead view of a set of overlapping antennas 900 in one direction.

FIG. 10 is an overhead view of an antenna array 1000.

FIG. 11 is an overhead view of a polar antenna array 1100.

FIG. 12A is an overhead view of a Baccarat table 1200, and FIG. 12B is an overhead view of a portion of the Baccarat table 1200 showing a corresponding portion of an antenna array 1202.

FIG. 13 is an overhead view of a roulette table 1300 having an antenna array 1302.

FIG. 14 is an overhead view of the antennas 104 and 106 of FIG. 1

DETAILED DESCRIPTION

Described herein are techniques for location determination of RFID tags. In the following description, for purposes of explanation, numerous examples and specific details are set forth in order to provide a thorough understanding of the present invention. It will be evident, however, to one skilled in the art that the present invention as defined by the claims may include some or all of the features in these examples alone or in combination with other features described below, and may further include modifications and equivalents of the features and concepts described herein.

In the following description, various methods, processes and procedures are detailed. Although particular steps may be described in a certain order, such order is mainly for convenience and clarity. A particular step may be repeated more than once, may occur before or after other steps (even if those steps are otherwise described in another order), and may occur in parallel with other steps. A second step is required to follow a first step only when the first step must be completed before the second step is begun. Such a situation will be specifically pointed out when not clear from the context.

In this document, the terms “and”, “or” and “and/or” are used. Such terms are to be read as having an inclusive meaning. For example, “A and B” may mean at least the following: “both A and B”, “at least both A and B”. As another example, “A or B” may mean at least the following: “at least A”, “at least B”, “both A and B”, “at least both A and B”. As another example, “A and/or B” may mean at least the following: “A and B”, “A or B”. When an exclusive-or is intended, such will be specifically noted (e.g., “either A or B”, “at most one of A and B”).

In this document, the terms “RFID tag”, “RFID gaming tag”, “RFID chip”, “RFID gaming chip”, “gaming chip”, and “gaming token” are used. Such terms are to be read as being broadly synonymous. (More precisely, an “RFID chip” may be used to refer to the integrated circuit components of the “RFID tag”, which also includes additional components such as an antenna, a rigid housing, etc. However, this document is mostly concerned with the broad usage for these terms.) The RFID tag responds to a radio frequency signal from the RFID reader, generally with its serial number or other identifier, enabling the RFID reader to obtain an inventory of the RFID tags in the vicinity. In a gaming context, the RFID gaming tags may be placed on, removed from, or moved around on a gaming table as bets and payouts, according to various game rules. The RFID gaming tags may be marked with a value identifier (e.g., $1).

FIG. 1 is a block diagram of an RFID system 100. The RFID system 100 includes a main antenna 102, a first set of antennas 104 a-104 d (collectively antennas 104), a second set of antennas 106 a-106 d (collectively antennas 106), a main RFID transmitter 108, a main RFID receiver 112, a first set of RFID receivers 114 a-114 d (collectively RFID receivers 114), a second set of RFID receivers 116 a-116 d (collectively RFID receivers 116), and a controller 120. In general, the main RFID transmitter 108 generates radio frequency energy that is radiated by the antenna 102 and received by any RFID tags; the responses from the RFID tags are then received by the antennas 102, 104 and 106. The responses from the RFID tags may be amplitude and phase information. The amplitude information may be in the form of received signal strength (RSSI) information, and the phase information may be in the form of in-phase (I) and quadrature (Q) information.

The RFID system 100 may be implemented as part of a gaming table (see, e.g., the roulette and Baccarat examples of FIGS. 12A-12B and 13). For example, the antennas 102, 104 and 106 may be embedded below the playing surface of the gaming table (in order to detect the locations of the RFID gaming tags during play on the gaming table), and the rest of the components of the RFID system 100 may be embedded within the structure of the gaming table. The gaming environment may have a number of gaming tables, each including an RFID system 100; the multiple RFID systems 100 may be connected to each other or to other components via a network.

The main antenna 102 is located under the playing surface of the gaming table. As part of playing games using the gaming table, RFID gaming tags are placed on, removed from, and moved around on the area above the main antenna 102. This area on the gaming table may be marked to show various subareas according to the particular game being implemented (see, e.g., the roulette and Baccarat examples of FIGS. 12A-12B and 13). The main antenna 102 may be implemented on a printed circuit board. The main RFID transmitter 108 and the main receiver 112 are coupled to the main antenna 102.

The antennas 104 are located under the playing surface of the gaming table and, like the main antenna 102, are associated with the gameplay area. The antennas 104 are oriented in a first direction. As shown in FIG. 1, the antennas 104 are oriented in the north-south (or y) direction. In other implementations, the antennas 104 may be oriented in other directions. The antennas 104 may be implemented on a printed circuit board, or as a layer of a multilayer circuit board that also includes the main antenna 102. The RFID receivers 114 are coupled to the antennas 104.

The antennas 106 are located under the playing surface of the gaming table and, like the main antenna 102, are associated with the gameplay area. The antennas 106 are oriented in a second direction that differs from the first direction of the antennas 104. As shown in FIG. 1, the antennas 106 are oriented in the east-west (or x) direction. In other implementations, the antennas 106 may be oriented in other directions. The antennas 106 may be implemented on a printed circuit board, or as a layer of a multilayer circuit board that also includes the main antenna 102 or the antennas 104. The RFID receivers 116 are coupled to the antennas 106.

The antennas 106 overlap the antennas 104; this overlap is shown using dotted lines in FIG. 1. This overlap generally allows both the antennas 104 and 106 to be associated with the gameplay area, in addition to the main antenna 102. In general, this allows at least three antennas (e.g., the main antenna 102, one of the antennas 104, and one of the antennas 106) to be associated with each location within the gameplay area. The spacing between each of the antennas 104, and the spacing between each of the antennas 106, may be adjusted as desired.

Collectively, the antennas 104 and 106 form what may be referred to as an antenna array. As shown in FIG. 1, the antennas 104 and 106 intersect at right angles. In other implementations, the antennas 104 and 106 may intersect at other angles. As shown in FIG. 1, the antennas 104 and 106 are rectangular in shape. In other implementations, the antennas 104 may have other shapes, such as ring shapes, pie shapes, curved shapes, rounded rectangular shapes, etc. The sizes of the antennas 104 and 106 may be adjusted as desired.

Four antennas 104 and four associated RFID receivers 114, and four antennas 106 and four associated RFID receivers 116, are shown in FIG. 1. These quantities may be adjusted as desired to cover larger or smaller areas and/or to increase or decrease spatial resolution.

In general, the main RFID receiver 112 is used to generate reference amplitude and phase information that the controller 120 uses when processing the amplitude and phase information from the antennas 104 and 106. Although the main RFID receiver 112 is shown as a separate component in FIG. 1, the main RFID receiver 112 may be a subcomponent of the main RFID transmitter 108.

The controller 120 generally controls the operation of the RFID system 100. The controller 120 may be connected to, or may be a component of, a computer (e.g., a personal computer). The controller 120 may connect to other components, or may itself include components, that implement other functions such as RFID tag identification, RFID tag location determination, game rules verification, etc. The controller 120 may access various data stores or databases such as a game rules database, an RFID tag database, etc.

The RFID system 100 generally operates as follows. The transmitter 108 generates a radio frequency signal that is transmitted by the main antenna 102. Any RFID gaming tags in the gameplay area respond to the radio frequency signal. The responses from the RFID gaming tags are received by the main receiver 112 (via the main antenna 102), at least one of the receivers 114 (via at least one of the antennas 104), and at least one of the receivers 116 (via at least one of the antennas 106). The controller 120 determines the position of each RFID gaming tag by correlating the responses received by each of the receivers. More details are provided with reference to FIG. 2.

According to another embodiment, the main receiver 112 may be omitted.

FIG. 2 is a flowchart of a method 200 of operating an RFID system (e.g., the RFID system 100 of FIG. 1). The method 200 may be controlled by a controller (e.g., the controller 120 of FIG. 1), for example, according to the execution of a computer program. In general, the method 200 describes a single RFID read cycle. Between read cycles, the RFID gaming tags are unpowered and do not send signals. During the RFID read cycle, each RFID gaming tag in the gameplay area responds. The RFID read cycle ends after each RFID gaming tag has responded. Thus, each RFID read cycle results in all the RFID gaming tags in the gameplay responding once but being read by multiple receivers.

At 202, the controller controls a main RFID transmitter (e.g., the main RFID transmitter 108 of FIG. 1) to generate an RFID inventory command. In general, the main RFID antenna is energized, and the RFID inventory command is one of a number of commands that may be included in the radio frequency energy generated by the main RFID transmitter. Further details of the RFID inventory command are provided below. A main antenna (e.g., the main antenna 102 of FIG. 1) coupled to the main RFID transmitter transmits the RFID inventory command. The main antenna is associated with an area on a gaming table that contains one or more RFID tags (e.g., RFID gaming tags).

At 204, each of the RFID tags responds to the RFID inventory command according to an anti-collision process. In general, the anti-collision process helps ensure that only one of the RFID tags is responding at a given time. Further details of the anti-collision process are provided below.

At 206, a main RFID receiver (e.g., the main RFID receiver 112 of FIG. 1) coupled to the main antenna receives a first set of responses from the RFID tags in the area in response to the RFID inventory command.

At 208, a first set of RFID receivers (e.g., the RFID receivers 114 of FIG. 1) coupled to a first set of antennas (e.g., the antennas 104 of FIG. 1) receives a second set of responses from the RFID tags in the area in response to the RFID inventory command. The first set of antennas is oriented in a first direction and is associated with the area on the gaming table.

At 210, a second set of RFID receivers (e.g., the RFID receivers 116 of FIG. 1) coupled to a second set of antennas (e.g., the antennas 106 of FIG. 1) receives a third set of responses from the RFID tags in the area in response to the RFID inventory command. The second set of antennas is oriented in a second direction that differs from the first direction, the second set of antennas overlaps the first set of antennas, and the first set of antennas and the second set of antennas intersect at a number of locations within the area.

As mentioned above, each RFID tag responds once, but each response is received by multiple antennas. For ease of description, these received responses are referred to as the “first set of responses”, the “second set of responses” and the “third set of responses” in 206-210 above. In general, each of the multiple antennas receives a particular response simultaneously.

Due to the anti-collision process, ideally only one of the RFID tags is responding at a given time, so controller is able to associate the responses received by each of the antennas at that given time with that one responding RFID tag. So generally 206-210 occur in parallel, with each RFID tag (ideally) responding at a given time and being detected by multiple receivers. For example, at a given time, the response from one RFID tag is received by the main RFID receiver, at least one of the first set of RFID receivers, and at least one of the second set of RFID receivers.

A brief description of the anti-collision process is as follows. The controller puts out a start of inventory command which includes a 5 bit cyclic redundancy check (CRC). This command also defines how many slots there are. The tag creates a random number and compares it to the particular slot number. If it matches then the tag responds with the 5 bit CRC from the command along with the 16 bit CRC of its serial number. If the controller receives this without detecting a collision then it resends the 5 bit CRC and the 16 bit CRC to the tags. The tag then responds by sending out its serial number and sets its flag so it does not respond to more queries until the flag is reset when RF power is removed. Thus, sending the CRC before the actual data speeds things as it is a shorter message to determine if there is a collision.

At 212, a controller (e.g., the controller 120 of FIG. 1) determines an identifier for each of the RFID tags using at least one of the first set of responses, the second set of responses, and the third set of responses. As discussed above, when a given tag responds with its serial number according to the anti-collision process, this response may be received by multiple RFID receivers (e.g., the main RFID receiver receives the first set of responses, including the response from the given tag; the first set of RFID receivers receives the second set of responses, including the response from the given tag; etc.). The controller may use one of the RFID receivers (e.g., the main RFID receiver) to determine the identifiers, and may use the information from the other RFID receivers for verification or confirmation purposes. When the current read cycle ends, the RFID tag loses power, the flag is cleared, and that RFID tag is free to respond during the next read cycle.

At 214, the controller determines a position of each of the RFID tags by correlating amplitude and phase information of the first set of responses with amplitude and phase information of the second set of responses and amplitude and phase information of the third set of responses. Further details of this correlation process are provided below. In general, the controller uses the information from the first set of responses to modify the second set of responses in order to determine one dimension of the position (e.g., the x dimension), and uses the information from the first set of responses to modify the third set of responses in order to determine another dimension of the position (e.g., the y dimension); the intersection of the x dimension and the y dimension then indicates the position of the RFID tag in the gaming area.

As an alternative in 214, the controller may determine a position of each of the RFID tags by correlating amplitude information only (not phase information) of the first set of responses with amplitude and phase information of the second set of responses and amplitude and phase information of the third set of responses. In this alternative, the main RFID receiver just performs excitation, and its amplitude information is used for normalization purposes; the phase information from the first and second sets of antennas is used to determine the position.

As another alternative in 214, the controller may determine a position of each of the RFID tags by correlating amplitude information of the first set of responses with amplitude information of the second set of responses and amplitude information of the third set of responses.

The controller may perform 212-214 in parallel, or may perform 214 prior to 212.

Once all of the RFID tags have responded, the current read cycle is complete. When the controller performs the next read cycle, the controller performs the method 200 again.

The controller may then use the identifier and position of each of the RFID tags to perform other gaming functions such as verifying the amounts and placements of bets and payouts, verifying conformance of the RFID tag placements with various game rules, etc. Further details of these gaming functions are provided below.

As discussed above with reference to FIG. 1, an alternative embodiment omits the main RFID receiver. In such an embodiment, the controller determines the identifier (see 212) and the position (see 214) without using the first set of responses.

FIG. 3 is a graph showing a plot 300 of amplitude and phase information detected by one of the antennas 104 or 106 (see FIG. 1) as the chip is moved across it. The plot 300 represents the amplitude and phase information resulting from the response of a single RFID tag at each given x position. For visualization purposes, imagine that we are using the antenna 104 a (see FIG. 1) to detect the RFID tag at each position. In FIG. 3, the antenna 104 a has a width of approximately 2 inches, positioned between approximately 1.8 and 3.8 inches from the zero position 302. So imagine that the RFID tag starts at the zero position 302. This is outside the antenna 104 a and has a zero amplitude, corresponding to the RFID tag not being detected. The left side of the antenna 104 a begins at 304. As the RFID tag moves from 302 to 304, the antenna 104 a detects the amplitude of the response from the RFID tag, which increases as the RFID tag nears the antenna 104 a. (The amplitude shown in FIG. 3 is normalized using the response received by the main antenna 102 of FIG. 1.) Note that the amplitude of the plot 300 is negative between 302 and 304; this is due to the comparison of the phase information for the RFID tag detected by the main antenna 102 versus the antenna 104 a. Specifically, the phase information detected by the main antenna 102 is out-of-phase with the phase information detected by this antenna 104 a; this out-of-phase result is shown as the negative amplitude in FIG. 3. The negative phase information indicates that the RFID tag is detected outside of the antenna 104 a.

When the RFID tag reaches 304 (directly above the left-side loop of the antenna 104 a), the amplitude information is zero, corresponding to the RFID tag not being detected. As the RFID tag moves toward the center of the antenna 104 a, the amplitude increases, reaching a maximum of about 0.75 at 306. Since the phase information detected by the main antenna 102 is in-phase with the phase information detected by the antenna 104 a, this indicates that the RFID tag is inside both antennas, and is shown by the positive amplitude curve of the plot 300 between 304 and 308. As the RFID tag moves toward the right-side loop of the antenna 104 a, the amplitude decreases down to zero at 308.

As the RFID tag continues past 308, the antenna 104 a detects that the amplitude information increases (negatively) for a bit before returning to zero at 310. As before, the amplitude is shown as a negative value due to the phase information comparison between the main antenna 102 and the antenna 104 a. Since the RFID tag is inside the main antenna 102 but outside the antenna 104 a between 308 and 310, the comparison result is out-of-phase, and the amplitude of the plot 300 is shown as a negative between 308 and 310.

As an example, imagine that the antenna 104 a detects an amplitude of 0.2. Using just the amplitude information, the RFID tag could be at the position corresponding to one of six points: 320, 322, 324, 326, 330 or 332. If the phase information indicates out-of-phase, the RFID tag could be at the position corresponding to one of the four points 320, 322, 324 or 326. If the phase information indicates in-phase, the RFID tag could be at the position corresponding to one of the two points 330 or 332. Next, FIG. 4 shows how adjacent antennas can be used to narrow these multiple positions down to a single position.

FIG. 4 is a graph showing plots 400 and 402 of amplitude and phase information detected by two of the antennas 104, or by two of the antennas 106 (see FIG. 1). For illustrative purposes, assume that the plot 400 corresponds to the signal received by the antenna 104 a, similar to the plot 300 (see FIG. 3); the plot 402 corresponds to the signal received by the nearby antenna 104 b. The antennas 104 a and 104 b may be referred to as adjacent antennas. The plots 400 and 402 are otherwise similar to the plot 300 (see FIG. 3).

Due to the distance between the antennas 104 a and 104 b, there is some overlap among the plots 400 and 402. This overlap provides the RFID system 100 (see FIG. 1) with the ability to correlate the amplitude and phase information detected by each antenna with the position of the RFID tag.

Returning to the example discussed above regarding FIG. 3, imagine that the antenna 104 a detects an in-phase amplitude of 0.2, which indicates the RFID tag could be at the position corresponding to one of two points: 430 or 432. Imagine that the antenna 104 b detects an out-of-phase amplitude of 0.2, which indicates that the RFID tag could be at the position corresponding to one of four points: 450, 452, 454 or 456. By correlating these measurements, the RFID system 100 determines that the position of the RFID tag is the one that corresponds to point 432 (antenna 104 a) and point 450 (antenna 104 b); on the gaming table, this position is slightly inside the right-hand side of the antenna 104 a. (Note that FIG. 4 corresponds to a side view of the antenna 104 a, so in the overhead view of FIG. 1, the position of the RFID tag corresponds to one on a line slightly inside the right-hand side of the antenna 104 a.) Next, FIG. 5 shows how to extend this example to two dimensions.

FIG. 5 is an overhead view of the antennas 104 and 106 of FIG. 1. Continuing the thought experiment discussed above regarding FIG. 4, imagine that the RFID system 100 uses the antennas 104 to determine that the RFID tag is on a position corresponding to the line 502, due to the amplitude and phase information detected by the antennas 104 a and 104 b. Further imagine that the RFID system 100 uses the antennas 106 to determine that the RFID tag is on a position corresponding to the line 504, due to the amplitude and phase information detected by the antennas 106 a and 106 b (in a manner similar to that discussed above regarding the antennas 104 a and 104 b). The RFID system 100 is then able to determine the position of the RFID tag as the intersection of the lines 502 and 504, position 506. In this manner, the RFID system 100 is able to determine the positions of one or more RFID tags in the vicinity of the antenna array.

FIG. 6 is a block diagram of an RFID system 600. The RFID system 600 shows a specific implementation of the RFID system 100 (see FIG. 1). The RFID system 600 includes a main antenna 602, a first set of antennas 604 a-604 d (collectively antennas 604), a second set of antennas 606 a-606 d (collectively antennas 606), an RFID reader 608, a main RFID receiver 612, a first set of RFID receivers 614 a-614 d (collectively RFID receivers 614), a second set of RFID receivers 616 a-616 d (collectively RFID receivers 616), and controllers 620 a and 620 b (collectively controllers 620). These components are similar to the components discussed above regarding the RFID system 100 (see FIG. 1). The RFID system 600 also includes an oscillator 630, a signal divider 632, microprocessors 640 a-640 i (collectively microprocessors 640), a dual-directional coupler 650, and a diode detector 652.

The RFID reader 608 includes an RFID transmitter and a RFID receiver. The RFID transmitter is similar to the main RFID transmitter 108 (see FIG. 1). The RFID receiver enables the RFID reader 608 to read the identifiers of the RFID tags, if so desired. The RFID reader 608 may be a “stock” or “off the shelf” RFID reader. The RFID reader 608 generates a radio frequency signal that is provided to the dual directional coupler 650. The RFID reader 608 may also read the identifiers of the responses from any RFID tags in the area.

The oscillator 630 generates a first local oscillator signal at a desired frequency. For the RFID system 600, the RFID tags are designed to operate at a frequency of 13.56 MHz. This frequency may be adjusted as desired in other embodiments. The oscillator 630 provides this first local oscillator signal to the receivers 612, 614 and 616 (as also shown in FIG. 7), and to the signal divider 632.

The signal divider 632 divides the first local oscillator signal from the oscillator 630 in order to generate a second local oscillator signal. For the RFID system 600, the RFID tags are designed to operate with a modulation frequency of 424 kHz. Thus, the signal divider 632 divides the 13.56 MHz signal by 32 to get 424 kHz. The modulation frequency may be adjusted as desired in other embodiments. The signal divider 632 provides this second local oscillator signal to the receivers 612, 614 and 616 (as also shown in FIG. 7).

The microprocessors 640 process the amplitude and phase information from the receivers 612, 614 and 616, and provide the amplitude and phase information from each of the receivers to the controller 620 b. The microprocessors 640 receive an enable signal from the controller 620 b to selectively enable them.

The dual-directional coupler 650 generally couples the RFID reader 608, the main antenna 602, the main RFID receiver 612, and the controller 620 b (via the diode detector 652). The dual-directional coupler 650 couples the radio frequency energy transmitted by the RFID reader 608 to the main antenna 602, and directs a portion of the transmitted radio frequency energy to the controller 620 b (via the diode detector 652). The dual-directional coupler 650 couples the radio frequency energy received by the main antenna 602 to the RFID reader 608, and directs a portion of the received radio frequency energy to the main RFID receiver 612.

The diode detector 652 generally functions as an envelope detector. The controller 620 b uses the output of the diode detector to determine the time at which a tag may be responding. This allows the controller 620 b to have the receivers 614 and 616 start sampling the antenna signals.

The controllers 620 generally control the operation of the RFID system 600, as discussed above regarding the controller 120 (see FIG. 1) and the method 200 (see FIG. 2). The controller 620 a generally controls the RFID reader 608, and processes the data collected by the controller 620 b in order to determine the positions of the RFID tags. The controller 620 a may connect to the RFID reader 608 via an Ethernet connection, and may connect to the controller 620 b via a universal serial bus (USB) connection. The controller 620 a may be implemented with a computer (e.g., a personal computer) that is connected via a network to other devices. The controller 620 b generally collects the amplitude and phase information received by the receivers 612, 614 and 616. The controller 620 b may be implemented with a microprocessor or a programmable logic device.

The RFID system 600 generally operates as follows. The controller 620 a instructs the RFID reader 608 to transmit an inventory command. The RFID reader 608 turns on its radio frequency output and sends a signal to the RFID tags (e.g., by amplitude modulating the carrier signal of its radio frequency output). The dual-directional coupler 650 directs a portion of this signal to the diode detector 652. The controller 620 b monitors the output of the diode detector 652 in order to determine when to have the receivers start sampling for an RFID tag which may be responding to the RFID reader 608. At this time, the controller 620 b instructs the microprocessors 640, using the enable signal, to begin sampling the RSSI information. When the controller 620 b determines the end of the RFID tag response, the controller 620 b uses the enable signal to instruct the microprocessors 640 to sample the I and Q levels from the receivers 612, 614 and 616, and to process the RSSI information to determine the data returned by the RFID tag. (This data is generally the serial number of the RFID tag, in response to the inventory command.) The I and Q information determine the phase of the modulated second local oscillator signal of the response from the RFID tag (e.g., at 424 kHz). Note that the phase of the modulated second local oscillator signal is indeterminate when using a single one of the receivers 614 or 616. However, by comparing the phase detected by one of the receivers 614 or 616 and the phase detected by the receiver 612, the controller 620 b is able to determine whether or not the RFID tag is inside of, or outside of, a given antenna loop.

The oscillator 630 provides the first local oscillator signal (e.g., 13.56 MHz) to the receivers 612, 614 and 616, and to the signal divider 632. The signal divider 632 generates the second local oscillator signal (e.g., 424 kHz) and provides this second local oscillator signal to the receivers 612, 614 and 616. The RFID tags respond to the inventory command by load modulating a subcarrier signal onto the carrier signal transmitted by the RFID reader 608. The receivers 612, 614 and 616 determine the modulated subcarrier signal from the RFID tags by first mixing the antenna signal first with the first local oscillator signal. After filtering and amplifying, the signal is mixed with the second local oscillator signal to demodulate the modulated subcarrier signal to baseband to determine the I and Q components.

The controller 620 b analyzes the data from the receivers 614, and the data from the receiver 612, to determine the location of the RFID tag on the x axis. Similarly, the controller 620 b analyzes the data from the receivers 616, and the data from the receiver 612, to determine the location of the RFID tag on the y axis. The controller 620 b may use the RSSI from the receiver 612 to normalize the signals received from the other receivers 614 and 616 so that higher fidelity position information can be attained.

Although four sets of antennas 604 and antennas 606 (and their associated receivers 614 and 616) are shown, these numbers may be adjusted as desired. Similarly, the shapes of the antennas 604 and 606 may be adjusted.

FIG. 7 is a block diagram of a receiver 700. The receiver 700 may be a specific implementation for one or more of the receivers 612, 614 or 616 (see FIG. 6). The receiver 700 includes a mixer 702, a band-pass filter 704, an amplifier 706, a band-pass filter 708, a limiting amplifier 710, mixers 712 a and 712 b, a phase shifter 714, resistors 716 a and 716 b, and capacitors 718 a and 7128 b.

The receiver 700 is connected to one of the antennas (e.g., one of the antennas 602, 604 or 606 of FIG. 6). The receiver 700 receives a first local oscillator (LO) signal 730 (e.g., at 13.56 MHz) from the oscillator 630 (see FIG. 6), and receives a second local oscillator signal 732 (e.g., at 424 kHz) from the signal divider 632 (see FIG. 6).

The mixer 702 mixes the radio frequency signal received by the antenna (e.g., one of the antennas 602, 604 or 606 of FIG. 6) with the first local oscillator signal 730, in order to generate a modulated subcarrier signal 734 (e.g., at 424 kHz). (The subcarrier signal 734 is a modulated subcarrier signal due to the radio frequency energy from the RFID reader (e.g., 608 in FIG. 6) being modulated by the RFID tag in the area.)

The band-pass filter 704 performs band-pass filtering on the modulated subcarrier signal 734 to reduce the noise, and generates a modulated subcarrier signal 736. The band-pass filter 704 has a center frequency around the expected frequency of the subcarrier signal (e.g., 424 kHz).

The amplifier 706 amplifies the modulated subcarrier signal 736, and generates a modulated subcarrier signal 738. The band-pass filter 708 performs band-pass filtering on the modulated subcarrier signal 738 to further reduce the noise, and generates a modulated subcarrier signal 740. The band-pass filter 708 has a center frequency around the expected frequency of the subcarrier signal (e.g., 424 kHz).

The limiting amplifier 710 drives the modulated subcarrier signal 740 into limiting (e.g., by having a high gain) so that the I and Q phase signals are independent of signal amplitude, resulting in a modulated subcarrier signal 742. The limiting amplifier 710 also outputs a RSSI signal 744 that is proportional to the level of the modulated subcarrier signal 740 (e.g., in dB). The RSSI signal 744 is then provided to the controller 620 b of FIG. 6, and corresponds to the RSSI or amplitude information discussed above.

The mixer 712 a mixes the modulated subcarrier signal 742 with the second local oscillator signal 732 in order to extract a modulated signal 746. The modulated signal 746 corresponds to the modulation of the subcarrier signal (e.g., at 424 kHz) performed by the RFID tag in the area. The resistor 716 a and the capacitor 718 a form a low-pass filter that performs low-pass filtering on the modulated signal 746, resulting in an in-phase (I) signal 748. The in-phase signal 748 is then provided to the controller 620 b of FIG. 6, and corresponds to the in-phase (I) signal component discussed above.

The phase shifter 714 performs phase-shifting by 90 degrees on the second local oscillator signal 732 to generates a phase-shifted second local oscillator signal 733.

The mixer 712 b mixes the modulated subcarrier signal 742 with the phase-shifted second local oscillator signal 733 in order to create a demodulated signal 750. The demodulated signal 750 corresponds to an unfiltered quadrature (Q) signal. The resistor 716 b and the capacitor 718 b form a low-pass filter that performs low-pass filtering on the demodulated signal 750, resulting in a quadrature (Q) signal 752. The quadrature signal 752 is then provided to the controller 620 b of FIG. 6, and corresponds to the quadrature (Q) signal component discussed above.

As discussed above, the controllers 620 (see FIG. 6) are able to determine whether a given RFID tags is inside of, or outside of, one or more of the antennas 604 and 606 by comparing the I and Q components received by that antenna with the I and Q components received by the main antenna 602.

According to another embodiment, instead of the receiver 700, the receiver may be implemented as a software defined radio. In general, a software defined radio samples the signal from the antenna with a high speed analog to digital converter, then processes the signals digitally, in order to detect the amplitude and phase.

FIG. 8 is a block diagram of an RFID system 800. The RFID system 800 shows a specific implementation of the RFID system 100 (see FIG. 1). The RFID system 800 includes a main antenna 802, a first set of antennas 804 a-804 d (collectively antennas 804), a second set of antennas 806 a-806 d (collectively antennas 806), an RF transmitter 808, a main RFID receiver 812, a first set of RFID receivers 814 a-814 d (collectively RFID receivers 814), a second set of RFID receivers 816 a-816 d (collectively RFID receivers 816), controllers 820 a and 820 b (collectively controllers 820), an oscillator 830, and a signal divider 832. These components are similar to the components discussed above regarding the RFID system 100 (see FIG. 1) or the RFID system 600 (see FIG. 6). The RFID system 800 also includes a directional coupler 850.

The RFID system 800 is similar to the RFID system 600 (see FIG. 6), with the main differences being replacing the RFID reader 608 (see FIG. 6) with the RF transmitter 808, replacing the controller 620 b with the controller 820 b, and replacing the dual-directional coupler 650 with the directional coupler 850. In brief, the RFID system 600 (see FIG. 6) is directed to using a “stock” or “off the shelf” RFID reader (the RFID reader 608), and the RFID system 800 is directed to using a controllable RF transmitter (the RF transmitter 808).

The oscillator 830 generates a first local oscillator signal at a desired frequency. For the RFID system 800, the RFID tags are designed to operate at a frequency of 13.56 MHz. This frequency may be adjusted as desired in other embodiments. The oscillator 830 provides the first local oscillator signal to the receivers 812, 814 and 816. The receivers 812, 814 and 816 may be implemented in a manner similar to the receiver 700 (see FIG. 7), in which case this signal corresponds to the first local oscillator signal 730.

The signal divider 832 divides the first local oscillator signal from the oscillator 830 in order to generate a second local oscillator signal. For the RFID system 800, the RFID tags are designed to operate with a modulation frequency of 424 kHz. Thus, the signal divider 832 divides the 13.56 MHz signal by 32 to get 424 kHz. The modulation frequency may be adjusted as desired in other embodiments. The signal divider 832 provides this second local oscillator signal to the receivers 812, 814 and 816 (as also shown in FIG. 7), in which case this signal corresponds to the second local oscillator signal 732.

The directional coupler 850 generally couples the RF transmitter 808, the main antenna 802 and the main RFID receiver 812. The directional coupler 850 couples the radio frequency energy transmitted by the RF transmitter 808 to the main antenna 802. The directional coupler 850 couples the radio frequency energy received by the main antenna 802 to the RF transmitter 808, and directs the received radio frequency energy to the main RFID receiver 812.

The controllers 820 generally control the operation of the RFID system 800, as discussed above regarding the controller 120 (see FIG. 1) and the method 200 (see FIG. 2). The controller 820 a generally acts as an interface to the other components. The controller 820 a may connect to the controller 820 b via an Ethernet connection. The controller 820 a may be implemented with a computer (e.g., a personal computer) that is connected via a network to other devices. The controller 820 b generally controls the RF transmitter 808, collects the amplitude and phase information received by the receivers 812, 814 and 816, and processes the amplitude and phase information to determine the positions of the RFID tags. The controller 820 b may be implemented with a microprocessor or a programmable logic device.

The RFID system 800 generally operates as follows. The controller 820 b controls the RF transmitter 808 using a modulation signal 860. The RF transmitter 808 applies the modulation signal 860 to its RF carrier signal to command the tags (e.g., to transmit an inventory command using amplitude modulation of a subcarrier signal on the carrier signal of its radio frequency output). The directional coupler 850 directs this signal to the main antenna 802. The controller 820 b receives the amplitude and phase information (RSSI, I and Q) from the receivers 812, 814 and 816. (This data is generally the serial number of the RFID tag, in response to the inventory command.) The I and Q information determine the phase of the modulated subcarrier of the response from the RFID tag (e.g., at 424 kHz). Note that the phase of the modulated subcarrier is indeterminate when using a single one of the receivers 814 or 816. However, by comparing the phase detected by one of the receivers 814 or 816 and the phase detected by the receiver 812, the controller 820 b is able to determine whether or not the RFID tag is inside of, or outside of, a given antenna loop.

The oscillator 830 provides the first local oscillator signal (e.g., 13.56 MHz) to the receivers 812, 814 and 816, and to the signal divider 832. The signal divider 832 generates the second local oscillator signal (e.g., 424 kHz) and provides this signal to the receivers 812, 814 and 816. The RFID tags respond to the inventory command by load modulating a subcarrier signal onto the carrier signal transmitted by the RF transmitter 808. The receivers 812, 814 and 816 determine the modulated subcarrier signal from the RFID tags by mixing the detected subcarrier signal with the first local oscillator signal from the oscillator 830. The receivers 812, 814 and 816 demodulate the modulated subcarrier signal to baseband to determine the I and Q components.

The controller 820 b analyzes the data from the receivers 814, and the data from the receiver 812, to determine the location of the RFID tag on the x axis. Similarly, the controller 820 b analyzes the data from the receivers 816, and the data from the receiver 812, to determine the location of the RFID tag on the y axis. The controller 820 b may use the RSSI from the receiver 812 to normalize the signals received from the other receivers 814 and 816 so that higher fidelity position information can be attained.

Although four sets of antennas 804 and antennas 806 (and their associated receivers 814 and 816) are shown, these numbers may be adjusted as desired. Similarly, the shapes of the antennas 804 and 806 may be adjusted.

FIG. 9 is an overhead view of a set of overlapping antennas 900 in one direction. The overlapping of the antennas 900 increases the number of antennas that may receive the response from a given RFID tag, which increases the amount of data available to the RFID system and possibly increases the accuracy of the position determination of the RFID tag. The antennas 900 include antennas 900 a, 900 b, 900 c, 900 d, 900 e, 900 f and 900 g. The antennas 900 are associated with RFID receivers (not shown); these RFID receivers may be similar to the RFID receivers 114, 116 (see FIG. 1), 614, 616 (see FIG. 6), 700 (see FIG. 7), 814, or 816 (see FIG. 8). Note that the antennas 900 e, 900 f and 900 g are shown slightly offset, for illustrative clarity.

The antennas 900 may be used in place of one of the sets of antennas in a particular direction. For example, the antennas 900 may be used in place of the antennas 104 (see FIG. 1) for the x direction, or the antennas 106 (see FIG. 1) for the y direction. The antennas 900 may be used in place of the antennas 604 (see FIG. 6) for the x direction, or the antennas 606 (see FIG. 6) for the y direction. The antennas 900 may be used in place of the antennas 804 (see FIG. 8) for the x direction, or the antennas 806 (see FIG. 8) for the y direction.

The antennas 900 may be printed as dual layers on a printed circuit board. The number, and shape, of the antennas 900 may be adjusted as desired.

FIG. 10 is an overhead view of an antenna array 1000. The antenna array 1000 includes a first set of antennas 1004 a, 1004 b, 1004 c and 1004 d (collectively antennas 1004), and a second set of antennas 1006 a, 1006 b, 1006 c, 1006 d, 1006 e, 1006 f, 1006 g, 1006 h, 1006 i (collectively antennas 1006). As compared to other of the antenna arrays (e.g., the antennas 104 and 106 of FIG. 1), the antennas 1004 and 1006 do not intersect at right angles. The antenna array 1000 may be used in place of the antennas 104 and 106 (see FIG. 1), 604 and 606 (see FIG. 6), or 804 and 806 (see FIG. 8). The antennas 1004, the antennas 1006, or both, may be overlapping in a manner similar to that of the antennas 900 (see FIG. 9).

The number, and shape, of the antennas 1004 and 1006 may be adjusted as desired.

FIG. 11 is an overhead view of a polar antenna array 1100. The polar antenna array 1100 includes overlapping circular antennas 1104 a, 1104 b, 1104 c and 1104 d (collectively circular antennas 1104), and radial antennas 1106 a, 1106 b, 1106 c and 1106 d (collectively radial antennas 1106). The antennas 1104 are shown with dotted lines. As compared to other of the antenna arrays (e.g., the antennas 104 and 106 of FIG. 1), the antenna array 1100 does not generate x and y position information, but instead generates polar position information (e.g., magnitude and direction). The circular antennas 1104 are used to determine the distance from the center point 1110, and the radial antennas 1106 are used to determine the angle. The polar antenna array 1100 may be used in place of the antennas 104 and 106 (see FIG. 1), 604 and 606 (see FIG. 6), 804 and 806 (see FIG. 8), or the antenna array 1000 (see FIG. 10).

As an example, say that the system detects an RFID tag inside of the circular antenna 1104 d and outside of the circular antenna 1104 c. If the system further detects that RFID tag outside of the radial antenna 1106 a, the system determines that the position of the RFID tag is in the vicinity of the point 1110. The accuracy of the position determination can be determined according to the values of the signals detected, as discussed above regarding FIGS. 3-4 (or as further detailed below in the section Determining the Positions).

As an option, the circular antennas 1104 need not be overlapping. Similarly, the radial antennas 1106 need not be overlapping. As another option, the circular antennas 1104 can be annular or ring-shaped, and overlapping or non-overlapping or partially overlapping.

FIG. 12A is an overhead view of a Baccarat table 1200, and FIG. 12B is an overhead view of a portion of the Baccarat table 1200 showing a corresponding portion of an antenna array 1202. In FIG. 12A, the entire antenna array 1202 is present, but not shown. In FIG. 12B, the antenna array 1202 includes a first set of antennas 1204 a, 1204 b, 1204 c, 1204 d, 1204 e, 1204 f, 1204 g and 1204 h (collectively antennas 1204) and a second set of antennas 1206 a, 1206 b, 1206 c and 1206 d (collectively antennas 1206). The Baccarat table 1200 also includes one or more main antennas (not shown), similar to the main antenna 102 (see FIG. 1). For one main antenna, it may surround the entire playing area of the Baccarat table 1200. For two main antennas, each may cover a portion of the Baccarat table 1200. For example, one may surround the betting positions on the left-hand portion of the Baccarat table 1200 (positions 7-12), and the other may surround the betting positions on the right-hand portion of the Baccarat table 1200 (positions 1-6). Alternatively, the main antenna can define a bounded area for some (or all) of a single type of bet (e.g. player or banker). Such an array can be used to track individual bets within the bounded area.

The antennas 1204 and 1206 (and the main antenna) are connected to RFID readers (not shown), in a manner similar to the RFID readers 114 or 116 (see FIG. 1). The antennas 1204 are wider at one end than at the other end. The antennas 1206 are slightly bent or curved, in order to conform to the Baccarat table 1200.

As an option, the antennas 1204, the antennas 1206, or both may be overlapping in a manner similar to that of the antennas 900 (see FIG. 9).

FIG. 13 is an overhead view of a roulette table 1300 having an antenna array 1302. The antenna array 1302 includes a first set of antennas 1304 a, 1304 b, 1304 c, 1304 d and 1304 e (collectively antennas 1304) and a second set of antennas 1306 a, 1306 b, 1306 c, 1306 d, 1306 e, 1306 f, 1306 g, 1306 h, 1306 i, 1306 j, 1306 k, 1306 l, 1306 m and 1306 n (collectively antennas 1306). The roulette table 1300 also includes a main antenna (not shown), similar to the main antenna 102 (see FIG. 1), that surrounds the playing area. The antennas 1304 and 1306 (and the main antenna) are connected to RFID readers (not shown), in a manner similar to the RFID readers 114 or 116 (see FIG. 1).

As an option, the antennas 1304, the antennas 1306, or both may be overlapping in a manner similar to that of the antennas 900 (see FIG. 9).

Finally, regarding the sizing of the antennas discussed herein, generally the width of each antenna should be less than the diameter of the RFID tags (or within around +/−0.5 inches of the diameter of the RFID tags), and the spacing between each antenna should also be less than the diameter of the RFID tags.

Reading the RFID Tags

As discussed above, the RFID reader (e.g., the RFID transmitter 108 of FIG. 1) sends an inventory command (e.g., 202 in FIG. 2) that the RFID tags respond to (e.g., 204 in FIG. 2). The RFID tags include anti-collision features to mitigate interference resulting when two or more RFID tags respond at the same time. One anti-collision feature is a pseudo-random selection of the slot in which they respond. Statistically, the different pseudo-random slots among a plurality of RFID tags helps prevent them from all responding at the same time.

Another anti-collision feature is the 5 bit CRC that is added to the inventory command which is sent from the tag to the reader along with the 16 bit CRC of the serial number of the tag. If the CRCs are not correct then there is likely a collision. When all the RFID tags have been read, the RFID reader stops radiating energy, which causes the RFID tags to clear their flags; with the cleared flags, the RFID tags are free to respond when the RFID reader begins radiating energy again when sending the next inventory command.

The RFID reader may implement a slotted Aloha system or a binary tree search. In the slotted Aloha system, the RFID reader broadcasts an initialization command and a parameter that the tags individually use to pseudo-randomly delay their responses. In the binary tree search, the RFID reader sends an initialization symbol and then transmits one bit of identification data at a time; only RFID tags with matching bits respond, and eventually only one RFID tag matches the complete identification string.

Each RFID tag may include 96 bits of identification information, which allows for 2̂96 total RFID tags to be individually identified by the system. The RFID tags may send their responses using Manchester encoding of their modulation on the carrier signal from the RFID reader.

As a result of the anti-collision features, the system can generally operate as if only one RFID tag is responding at a given time. This allows all of the receivers (e.g., the receivers 112, 114 and 116 of FIG. 1) that receive a response at a given time to associate together the respective responses received by each receiver. For clarity of illustration, the remainder of this document assumes that only one RFID tag is responding at a given time.

Determining the Positions

As discussed above, at least three receivers (e.g., the receiver 112, at least one of the receivers 114, and at least one of the receivers 116 of FIG. 1) of the RFID system (e.g., the RFID system 100) receive the response from a given RFID tag. For a given antenna in the x direction (e.g., the antenna 104 b), the RFID system can determine that the given RFID tag is inside of, or outside of, the given antenna by comparing the signal phases between the given antenna in the x direction and the main antenna (e.g., the main antenna 102). Similarly, for a given antenna in the y direction (e.g., the antenna 106 b), the RFID system can determine that the given RFID tag is inside of, or outside of, the given antenna by comparing the signal phases between the given antenna in the y direction and the main antenna. When the RFID system determines that the given RFID tag is inside of both the x direction antenna and the y direction antenna, the RFID system determines the position of the given RFID tag on the gaming table as being at the position where those two antennas intersect. In a simple case, the RFID system assumes the position is at the midpoint of the intersection.

When the RFID system determines that the given RFID tag is outside of either the x direction antenna or the y direction antenna, the RFID system needs to determine which direction outside. As an example in the x direction, if the given RFID tag is detected outside of the antenna 104 b, the given RFID tag may be to the left of, or the right of, the antenna 104 b. At this point, the RFID system looks at the responses received by the antennas adjacent to the antenna 104 b (e.g., the antennas 104 a and 104 c). If the antenna 104 a received the response and the antenna 104 c did not, then the RFID system determines the position of the given RFID tag as to the left of the antenna 104 b. Similarly, if the antenna 104 c received the response and the antenna 104 a did not, then the RFID system determines the position of the given RFID tag as to the right of the antenna 104 b. In a simple case, the RFID system assumes the position is at the midpoint between the two antennas (104 b and 104 a, or 104 b and 104 b). In the y direction, a similar result occurs.

Interpolation

Instead of assuming the position at the midpoint, as discussed above, the RFID system (e.g., the RFID system 100 of FIG. 1) may interpolate the position based on the amplitude of the received signal. For example, the RFID system may store the plot 300 of FIG. 3 as a lookup table (e.g., in the controller 120). TABLE 1 is an example lookup table with 7 entries, corresponding to 7 segments of the plot 300 (where segment 1 is from point 302 down to the maximum negative value of the plot 300; segment 2 is from that point up to point 304; segment 3 is from point 304 up to near the maximum positive value of the plot 300; segment 4 is the portion around the maximum positive value; segment 5 is from near the maximum value down to point 308; segment 6 is from point 308 down to the maximum negative value of the plot 300; and segment 7 is from the maximum negative value up to point 310):

TABLE 1 Segment Amplitude Position 1 From 0 to −0.25 0.5 (approx. point 320) 2 From −0.25 to 0 1.4 (approx. point 322) 3 From 0 to 0.7 1.9 4 Above 0.7 2.5 5 From 0.7 to 0 3.2 6 From 0 to −0.25 3.75 (approx. point 324) 7 From −0.25 to 0 4.2 (approx. point 326)

(Since the plot 300 is symmetric, the data in TABLE 1 may be reduced to 4 entries, as offsets around a center point.) As discussed above, there may be multiple points for a given amplitude detected by a single antenna, so the RFID system uses adjacent antennas in order to eliminate the unlikely points.

The position data in TABLE 1 corresponds to the midpoint of each segment of the plot 300. Instead of using the midpoint when the amplitude falls anywhere within the appropriate range, the RFID system can interpolate using the exact amplitude. For example, if the amplitude is at −0.125 and (using adjacent antennas) the position is determined to be within the first segment, instead of using 0.5 as the position, the RFID system interpolates the position as halfway to 0.5, which is 0.25. The RFID system may use linear interpolation.

The number of entries in the lookup table may be increased, or decreased, as desired. As more entries appear in the lookup table, the interpolation becomes more accurate to the actual position.

In general, the values in TABLE 1 are applicable to the uniform-width antennas, such as in FIG. 5. For other antennas, such as the radial antennas 1106 of FIG. 11, or the antennas of FIG. 12B, the values in the corresponding lookup tables may be determined empirically.

Integration with Game Rules

The RFID system (e.g., the RFID system 100 of FIG. 1) may use the determined RFID information (e.g., the detected RFID tag identifiers and positions) to control various events in the gaming environment. In general, these events are managed according to game rules, and different game rules apply in different gaming circumstances (referred to as game states). When the RFID system detects a violation of the game rules, the RFID system may generate an alert. The RFID system may use the RFID readers described herein to determine the detected RFID tag identifiers and positions (generally referred to as chip data), and may use an instrumented card shoe to determine the values of cards dealt (generally referred to as card data).

In general, the game states are tailored to a particular game. For example, Baccarat may have the following game states: pre-game, new game, bets locked, payout, and end of game. In the pre-game state, the RFID system is not monitoring RFID tag identifiers or locations. In the new game state, the RFID system may track RFID tag identifiers and locations (and may display and record the resulting data), but since the game rules allow chips to be freely moved around in this state, no illegal move alerts are generated. (An exception may be made for detecting an illegal chip, which may result an illegal chip alert.) In the bets locked state, chip movements are not allowed, so any RFID tag movements detected may result in an alert. In the payout state, the RFID system monitors that the correct payout amounts are made to the correct locations, and that the correct collections are made from the correct locations, by correlating the RFID tags placed at (or removed from) the various locations. In the end of game state, the RFID system logs the end of the current game, and returns to the new game state for the next game.

A particular game state may include one or more sub-states (that may also be referred to as game states). For example, in blackjack, the dealer is obligated to deal another card to the dealer's hand depending upon the point total of the dealer's hand (e.g., 17). So within the game state of “deal cards to dealer's hand”, there is a sub-state of “deal another card” and a sub-state of “deal no more cards”. Similar states and sub-states exist for each hand. Similarly, if the dealer is dealt an initial Blackjack, the RFID system may transition from the “deal” state to the “collection/payout” state. As another example, Baccarat has a variety of sub-states within the gameplay state that each play position transitions between, depending upon the cards dealt. For example, the transition from play to payout can be staggered for each player.

The RFID system is particularly helpful during the collection and payout states. For example, the RFID system determines that Location 1 is a winner, and Location 2 is a loser, based on the game results. The RFID system knows the identifiers of the RFID tags associated with the locations, and verifies that additional chips corresponding to a correct payout are made to Location 1, and that the chips associated with Location 2 are collected.

Further information regarding the game rules and game states can be found in U.S. Application Pub. No. 2015/0312517 and U.S. Application Pub. No. 2016/0217645, which are incorporated herein by reference.

Grouping

The RFID system (e.g., the RFID system 100 of FIG. 1) may associate RFID tags that have similar positions into a single group. (These similar positions refer to the x-y plane; e.g., two RFID tags stacked on top of each other will have similar x-y positions but different z positions.) For example, if the determined positions of two RFID tags are less than approximately 0.75× diameters, the RFID system may consider those two RFID tags to be associated in a group. For example, for RFID tags having a diameter of 1.5 inches, a group results when two RFID tags are within about 1.125 inches. Similar groups may be formed from adjacent stacks of RFID tags. The controller may then consider that group of RFID tags as a single unit. For example, instead of interpreting a first RFID tag and a nearby second RFID tag as two separate bets (e.g., $100 and $200), the controller groups the two RFID tags as a single bet (e.g., $300). Generally, the RFID system may consider a set of RFID tags to be a group when the position of each RFID tag in the set is within a defined range (e.g., 0.75× diameters) of at least one other RFID tag in the set. For example, a “stack” of RFID tags will have similar positions (e.g., much less than 0.75× diameters), so the RFID system determines that stack to be a group. As another example, a “mound” of RFID tags may have positions that in the aggregate that are beyond the defined range, but as long as each RFID tag in the mound is within the defined range of at least one other RFID tag in the mound, the RFID system determines that mound to be a group.

Grouping may also be used in combination with the game rules (e.g., the game states and sub-states). For example, in Blackjack, a player is allowed to “double down” (to double the amount of the initial bet) in certain circumstances. In such a situation, the RFID system first uses the card data to determine that the double down is allowed. Second, the RFID system uses the RFID data to verify that an accurate doubled bet has been placed as a group with the initial bet. (The RFID system may determine the initial bet to be a first group and the doubled bet to be a second group.) Third, the RFID system uses the card data to determine if the player's hand is a winner or a loser; for a winner, the RFID system uses the chip data to verify that correct payouts have been placed as additional groups with the initial bet and the doubled bet; and for a loser, the RFID system uses the chip data to verify that all the groups (the initial bet and the doubled bet) are collected. If any of the data indicates a violation of the game rules, the RFID system may generate an alert.

The defined range that the RFID system uses to determine a group may be adjusted as desired. For example, when the defined range is 1.5× diameters, the RFID system determines that two adjacent stacks are a group.

FIG. 14 is an overhead view of the antennas 104 and 106 of FIG. 1. As compared to FIG. 5, FIG. 14 includes 5 RFID tags 1402, 1404, 1406, 1408 and 1410. Assume that each RFID tag takes 10 milliseconds (ms) to respond. Assuming no collisions, the read cycle takes 50 ms (10 ms per RFID tag): During this time, the main antenna (not shown) is energized, and the RFID reader connected to the antenna 104 a receives the responses from the RFID tags 1402 and 1404; the RFID reader connected to the antenna 104 b receives the responses from the RFID tags 1402, 1404 and 1406; the RFID reader connected to the antenna 104 c receives the responses from the RFID tags 1406, 1408 and 1410; and the RFID reader connected to the antenna 104 d receives the responses from the RFID tags 1408 and 1410. (These responses correspond to the “second set of responses” discussed above, at 208 in FIG. 2.) During the same time, the RFID reader connected to the antenna 106 a receives the responses from the RFID tags 1402 and 1408; the RFID reader connected to the antenna 106 b receives the responses from the RFID tags 1402, 1406 and 1408; the RFID reader connected to the antenna 106 c receives the responses from the RFID tags 1404, 1406 and 1410; and the RFID reader connected to the antenna 106 d receives the responses from the RFID tags 1404 and 1410. (These responses correspond to the “third set of responses” discussed above, at 210 in FIG. 2.)

Compare the above to a system that energizes each antenna separately (e.g., without a main antenna). In such a system, the RFID reader connected to the antenna 104 a takes 20 ms to perform a read (10 ms for each of the RFID tags 1402 and 1404), the RFID reader connected to the antenna 104 b takes 30 ms to perform a read (10 ms for each of the RFID tags 1402, 1404 and 1406), the RFID reader connected to the antenna 104 c takes 30 ms to perform a read (10 ms for each of the RFID tags 1406, 1408 and 1410), and the RFID reader connected to the antenna 104 d takes 20 ms to perform a read (10 ms for each of the RFID tags 1408 and 1410); so reading the x direction takes 100 ms (20+30+30+20). Similarly, reading the y direction also takes 100 ms, for a total read time of 200 ms. This is significantly more than the 50 ms discussed above.

Thus, the RFID systems described herein result in a notable improvement in read times as compared to existing systems that energize each antenna separately.

The above description illustrates various embodiments of the present invention along with examples of how aspects of the present invention may be implemented. The above examples and embodiments should not be deemed to be the only embodiments, and are presented to illustrate the flexibility and advantages of the present invention as defined by the following claims. Based on the above disclosure and the following claims, other arrangements, embodiments, implementations and equivalents will be evident to those skilled in the art and may be employed without departing from the spirit and scope of the invention as defined by the claims. 

What is claimed is:
 1. A system for determining locations of objects in a gaming environment, the system comprising: a main antenna associated with an area on a gaming table; a first plurality of antennas oriented in a first direction and associated with the area on the gaming table; a second plurality of antennas oriented in a second direction, wherein the second direction differs from the first direction, wherein the second plurality of antennas overlaps the first plurality of antennas, and wherein the first plurality of antennas and the second plurality of antennas intersect at a plurality of locations within the area; a main radio frequency identification (RFID) transmitter coupled to the main antenna; a first plurality of RFID receivers coupled to the first plurality of antennas; a second plurality of RFID receivers coupled to the second plurality of antennas; and a controller that controls the main RFID transmitter to generate an RFID inventory command, wherein each of a plurality of RFID tags in the area responds to the RFID inventory command according to an anti-collision process, wherein in response to the RFID inventory command, the first plurality of RFID receivers receives a first plurality of responses from the plurality of RFID tags, and the second plurality of RFID receivers receives a second plurality of responses from the plurality of RFID tags, wherein the controller determines an identifier for each of the plurality of RFID tags using at least one of the first plurality of responses and the second plurality of responses, and wherein the controller determines a position of each of the plurality of RFID tags by correlating phase information of the first plurality of responses, and phase information of the second plurality of responses.
 2. The system of claim 1, further comprising: a main RFID receiver coupled to the main antenna, wherein in response to the RFID inventory command, the main RFID receiver receives a third plurality of responses from the plurality of RFID tags, and wherein the controller determines the position of each of the plurality of RFID tags by correlating the phase information of the first plurality of responses, the phase information of the second plurality of responses, and phase information of the third plurality of responses.
 3. The system of claim 2, wherein the controller uses the phase information of the third plurality of responses to determine relative phase information for the first plurality of responses and relative phase information for the second plurality of responses, wherein the controller determines the position of each of the plurality of RFID tags by correlating the relative phase information of the first plurality of responses and the relative phase information of the second plurality of responses.
 4. The system of claim 1, wherein the controller determines the position of each of the plurality of RFID tags by correlating amplitude information and the phase information of the first plurality of responses, and the phase information of the second plurality of responses.
 5. The system of claim 1, further comprising: a main RFID receiver coupled to the main antenna, wherein in response to the RFID inventory command, the main RFID receiver receives a third plurality of responses from the plurality of RFID tags, and wherein the controller determines the position of each of the plurality of RFID tags by correlating amplitude information and the phase information of the first plurality of responses, the phase information of the second plurality of responses, and the phase information of the third plurality of responses.
 6. The system of claim 1, wherein for a particular RFID tag of the plurality of RFID tags, the controller simultaneously determines the identifier and the position of the particular RFID tag.
 7. The system of claim 1, wherein the RFID inventory command is a single RFID inventory command that results in the controller determining the identifiers and the positions of all the plurality of RFID tags.
 8. The system of claim 1, wherein the first plurality of antennas and the second plurality of antennas are overlapping and intersecting to define the position of each of the plurality of RFID tags in two dimensions within the area.
 9. The system of claim 1, wherein the first plurality of antennas and the second plurality of antennas intersect orthogonally and define the position of each of the plurality of RFID tags in an x dimension and a y dimension within the area.
 10. The system of claim 1, wherein the first plurality of antennas and the second plurality of antennas define the position of each of the plurality of RFID tags using polar coordinates within the area.
 11. A method of determining locations of objects in a gaming environment, the method comprising: generating, by a main radio frequency identification (RFID) transmitter coupled to a main antenna, an RFID inventory command, wherein the main antenna is associated with an area on a gaming table; responding, by each of a plurality of RFID tags in the area, to the RFID inventory command according to an anti-collision process; receiving, by a first plurality of RFID receivers coupled to a first plurality of antennas, a first plurality responses from the plurality of RFID tags in response to the RFID inventory command, wherein the first plurality of antennas is oriented in a first direction and is associated with the area on the gaming table; receiving, by a second plurality of RFID receivers coupled to a second plurality of antennas, a second plurality of responses from the plurality of RFID tags in response to the RFID inventory command, wherein the second plurality of antennas is oriented in a second direction that differs from the first direction, wherein the second plurality of antennas overlaps the first plurality of antennas, and wherein the first plurality of antennas and the second plurality of antennas intersect at a plurality of locations within the area; determining, by a controller, an identifier for each of the plurality of RFID tags using at least one of the first plurality of responses and the second plurality of responses; and determining, by the controller, a position of each of the plurality of RFID tags by correlating phase information of the first plurality of responses, and phase information of the second plurality of responses.
 12. The method of claim 11, further comprising: receiving, by a main RFID receiver coupled to the main antenna, a third plurality of responses from the plurality of RFID tags in response to the RFID inventory command, wherein the step of determining the position of each of the plurality of RFID tags comprises determining, by the controller, the position of each of the plurality of RFID tags by correlating the phase information of the first plurality of responses, the phase information of the second plurality of responses, and phase information of the third plurality of responses.
 13. The method of claim 12, further comprising: determining, by the controller, relative phase information for the first plurality of responses and relative phase information for the second plurality of responses by using the phase information of the third plurality of responses, wherein the step of determining the position of each of the plurality of RFID tags comprises determining, by the controller, the position of each of the plurality of RFID tags by correlating the relative phase information of the first plurality of responses and the relative phase information of the second plurality of responses.
 14. The method of claim 11, wherein the step of determining the position of each of the plurality of RFID tags comprises determining, by the controller, the position of each of the plurality of RFID tags by correlating amplitude information and the phase information of the first plurality of responses, and the phase information of the second plurality of responses.
 15. The method of claim 11, further comprising: receiving, by a main RFID receiver coupled to the main antenna, a third plurality of responses from the plurality of RFID tags in response to the RFID inventory command, wherein the step of determining the position of each of the plurality of RFID tags comprises determining, by the controller, the position of each of the plurality of RFID tags by correlating amplitude information and the phase information of the first plurality of responses, the phase information of the second plurality of responses, and the phase information of the third plurality of responses.
 16. The method of claim 11, wherein for a particular RFID tag of the plurality of RFID tags, the step of determining the identifier for the particular tag is performed simultaneously with the step of determining the position of the particular RFID tag.
 17. The method of claim 11, wherein the RFID inventory command is a single RFID inventory command, and wherein the steps of determining the identifier and determining the position for each of the plurality of RFID tags results in determining a plurality of identifiers and a plurality of positions for all of the plurality of RFID tags.
 18. The method of claim 11, wherein the first plurality of antennas and the second plurality of antennas are overlapping and intersecting to define the position of each of the plurality of RFID tags in two dimensions within the area.
 19. The method of claim 11, wherein the first plurality of antennas and the second plurality of antennas intersect orthogonally and define the position of each of the plurality of RFID tags in an x dimension and a y dimension within the area.
 20. The method of claim 11, wherein the first plurality of antennas and the second plurality of antennas define the position of each of the plurality of RFID tags using polar coordinates within the area. 